在iOS应用程序中拖动UITableViewCells时,可以顺畅地自动滚动UITableView

Question

我已经实现了一个系统来重新排序我的UITableViews中的单元格.一切都很好,除了我不能将细胞重新排序到iPhone屏幕上没有显示的位置.

所以我已经实现了一个条件来检查我是否需要滚动

NSArray *indexVisibles = [_tableView indexPathsForVisibleRows];
NSInteger indexForObject = [indexVisibles indexOfObject:indexPath];
if (indexForObject == NSNotFound){
     [_tableView scrollToRowAtIndexPath:indexPath
                                          atScrollPosition:UITableViewScrollPositionTop
                                                  animated:YES];
}

我的问题是动画不甜而干净.

我认为检查一个单元格是否显示的操作对于我的系统来说是非常巨大的,并且当我移动单元格时会产生一个小延迟,但是,我也不确定为什么当单元格隐藏时滚动是如此困难.

我已经改变UITableViewScrollPositionTop了UITableViewScrollPositionMiddle,现在它更好但是速度非常大所以总是滚动到我的UITableView的顶部.

我想慢慢来.

其他失败尝试:

选项1:

[UIView animateWithDuration:0.2
                                             animations:^{_tableView.contentOffset = CGPointMake(0.0, _tableView.contentOffset.y - 50);}
                                             completion:^(BOOL finished){ }];

但这有两个问题:

1. 运动仍然沉重
2. 拖动到第一个元素时,此元素为中间隐藏

选项2:

[UIView animateWithDuration: 1.0
                                             animations: ^{
                                                 [_tableView scrollToRowAtIndexPath:indexPath
                                                                   atScrollPosition:UITableViewScrollPositionMiddle
                                                                           animated:NO];
                                             }completion: ^(BOOL finished){

                                             }
                             ];

Answer 1

我用一个非常漂亮的解决方案解决了我的问题,所以我将通过三个简单的步骤解释如何做到这一点.

我从https://github.com/hpique/HPReorderTableView中使用了一些灵感,并在我自己的存储库中共享它https://github.com/enrimr/EMRReorderTableCells

A.管理手势识别

longPressGestureRecognized:

- (IBAction)longPressGestureRecognized:(id)sender {

    _reorderGestureRecognizer = (UILongPressGestureRecognizer *)sender;

    CGPoint location = [_reorderGestureRecognizer locationInView:_tableView];
    NSIndexPath *indexPath = [self getCellIndexPathWithPoint:location];

    UIGestureRecognizerState state = _reorderGestureRecognizer.state;
    switch (state) {
        case UIGestureRecognizerStateBegan: {

            NSIndexPath *indexPath = [_tableView indexPathForRowAtPoint:location];
            if (indexPath == nil)
            {
                [self gestureRecognizerCancel:_reorderGestureRecognizer];
                break;
            }

            // For scrolling while dragging
            _scrollDisplayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(scrollTableWithCell:)];
            [_scrollDisplayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

            // Check for the right indexes (between margins of offset
            if (indexPath.row >=_elementsOffset && indexPath.row < [_elements count]+_elementsOffset){

                if (indexPath) {
                    sourceIndexPath = indexPath;

                    id sourceElement = [_elements objectAtIndex:sourceIndexPath.row-_elementsOffset];

                    snapshot = [self createSnapshotForCellAtIndexPath:indexPath withPosition:location];
                }
            } else {
                sourceIndexPath = nil;
                snapshot = nil;
            }
            break;
        }

        case UIGestureRecognizerStateChanged: {

            [self calculateScroll:_reorderGestureRecognizer];

            if (sourceIndexPath != nil && indexPath.row >=_elementsOffset && indexPath.row < [_elements count]+_elementsOffset){
                [self updateSnapshotWithPosition:location];

                // Is destination valid and is it different from source?
                if (indexPath && ![indexPath isEqual:sourceIndexPath]) {
                    if (indexPath.row - sourceIndexPath.row <= 1){

                        id sourceElement = [_elements objectAtIndex:sourceIndexPath.row-_elementsOffset];
                        id targetElement = [_elements objectAtIndex:indexPath.row-_elementsOffset];

                        sourceIndexPath = [self exchangeElement:sourceElement byElement:targetElement];
                    }

                }
            }
            break;
        }

        case UIGestureRecognizerStateEnded:
        {
            // For scrolling while dragging
            [_scrollDisplayLink invalidate];
            _scrollDisplayLink = nil;
            _scrollRate = 0;


            // Check if it is the last element
            if (sourceIndexPath != nil){
                id element;
                if (indexPath.row <=_elementsOffset){
                    element = [_elements firstObject];
                } else if (indexPath.row > [_elements count]-1+_elementsOffset){
                    element = [_elements lastObject];
                } else {
                    element = [_elements objectAtIndex:indexPath.row-_elementsOffset];
                }  
            }

        }

        default: {
            // Clean up.
            [self deleteSnapshotForRowAtIndexPath:sourceIndexPath];

            [appDelegate startTimer];

            break;
        }
    }
}

gestureRecognizerCancel:

用于取消手势识别以完成重新排序操作.

-(void) gestureRecognizerCancel:(UIGestureRecognizer *) gestureRecognizer
{ // See: http://stackoverflow.com/a/4167471/143378
    gestureRecognizer.enabled = NO;
    gestureRecognizer.enabled = YES;
}

scrollTableWithCell:

当你处于桌子的极限时(上下),它被调用来进行滚动运动的方法

- (void)scrollTableWithCell:(NSTimer *)timer
{
    UILongPressGestureRecognizer *gesture = _reorderGestureRecognizer;
    const CGPoint location = [gesture locationInView:_tableView];

    CGPoint currentOffset = _tableView.contentOffset;
    CGPoint newOffset = CGPointMake(currentOffset.x, currentOffset.y + _scrollRate * 10);

    if (newOffset.y < -_tableView.contentInset.top)
    {
        newOffset.y = -_tableView.contentInset.top;
    }
    else if (_tableView.contentSize.height + _tableView.contentInset.bottom < _tableView.frame.size.height)
    {
        newOffset = currentOffset;
    }
    else if (newOffset.y > (_tableView.contentSize.height + _tableView.contentInset.bottom) - _tableView.frame.size.height)
    {
        newOffset.y = (_tableView.contentSize.height + _tableView.contentInset.bottom) - _tableView.frame.size.height;
    }

    [_tableView setContentOffset:newOffset];

    if (location.y >= 0 && location.y <= _tableView.contentSize.height + 50)
    {

        [self updateSnapshotWithPosition:location];
        NSIndexPath *indexPath = [self getCellIndexPathWithPoint:location];

        // CHeck if element is between offset limits.
        if (![indexPath isEqual:sourceIndexPath] &&
            indexPath.row >= _elementsOffset &&
            indexPath.row - _elementsOffset < [_elements count] &&
            sourceIndexPath.row >= _elementsOffset &&
            sourceIndexPath.row - _elementsOffset < [_elements count])
        {
            id sourceElement = [_elements objectAtIndex:sourceIndexPath.row-_elementsOffset];
            id targetElement = [_elements objectAtIndex:indexPath.row-_elementsOffset];
            [self exchangeElement:sourceElement byElement:targetElement];
            sourceIndexPath = indexPath;
        }
    }
}

B.快照管理

createSnapshotForCellAtIndexPath:withPosition

创建正在移动的单元格的快照(图像副本)的方法

-(UIView *)createSnapshotForCellAtIndexPath:(NSIndexPath *)indexPath withPosition:(CGPoint)location{
    UITableViewCell *cell = [_tableView cellForRowAtIndexPath:indexPath];

    // Take a snapshot of the selected row using helper method.
    snapshot = [self customSnapshoFromView:cell];

    // Add the snapshot as subview, centered at cell's center...
    __block CGPoint center = cell.center;
    snapshot.center = center;
    snapshot.alpha = 0.0;

    [_tableView addSubview:snapshot];
    [UIView animateWithDuration:0.25 animations:^{

        // Offset for gesture location.
        center.y = location.y;
        snapshot.center = center;
        snapshot.transform = CGAffineTransformMakeScale(1.05, 1.05);
        snapshot.alpha = 0.98;
        cell.alpha = 0.0;

    } completion:^(BOOL finished) {

        cell.hidden = YES;
    }];

    return snapshot;
}

customSnapshoFromView:

返回给定视图的自定义快照.*/

- (UIView *)customSnapshoFromView:(UIView *)inputView {

    // Make an image from the input view.
    UIGraphicsBeginImageContextWithOptions(inputView.bounds.size, NO, 0);
    [inputView.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    // Create an image view.
    snapshot = [[UIImageView alloc] initWithImage:image];
    snapshot.layer.masksToBounds = NO;
    snapshot.layer.cornerRadius = 0.0;
    snapshot.layer.shadowOffset = CGSizeMake(-5.0, 0.0);
    snapshot.layer.shadowRadius = 5.0;
    snapshot.layer.shadowOpacity = 0.4;

    return snapshot;
}

updateSnapshotWithPosition:

给定CGPoint,它会更改快照位置以显示您正在_tableView的正确位置移动的单元格

-(void)updateSnapshotWithPosition:(CGPoint)location{
    CGPoint center = snapshot.center;
    center.y = location.y;
    snapshot.center = center;
}

deleteSnapshotForRowAtIndexPath:

拖动完成后,您需要从_tableView中删除快照

-(void)deleteSnapshotForRowAtIndexPath:(NSIndexPath *)sourceIndexPath{
    UITableViewCell *cell = [_tableView cellForRowAtIndexPath:sourceIndexPath];
    cell.hidden = NO;
    cell.alpha = 0.0;

    [UIView animateWithDuration:0.25 animations:^{

        snapshot.center = cell.center;
        snapshot.transform = CGAffineTransformIdentity;
        snapshot.alpha = 0.0;
        cell.alpha = 1.0;

    } completion:^(BOOL finished) {
        [snapshot removeFromSuperview];
    }];
}

calculateScroll

-(void)calculateScroll:(UIGestureRecognizer *)gestureRecognizer{

    const CGPoint location = [gestureRecognizer locationInView:_tableView];

    CGRect rect = _tableView.bounds;
    // adjust rect for content inset as we will use it below for calculating scroll zones
    rect.size.height -= _tableView.contentInset.top;

    //[self updateCurrentLocation:gestureRecognizer];

    // tell us if we should scroll and which direction
    CGFloat scrollZoneHeight = rect.size.height / 6;
    CGFloat bottomScrollBeginning = _tableView.contentOffset.y + _tableView.contentInset.top + rect.size.height - scrollZoneHeight;
    CGFloat topScrollBeginning = _tableView.contentOffset.y + _tableView.contentInset.top  + scrollZoneHeight;

    // we're in the bottom zone
    if (location.y >= bottomScrollBeginning)
    {
        _scrollRate = (location.y - bottomScrollBeginning) / scrollZoneHeight;
    }
    // we're in the top zone
    else if (location.y <= topScrollBeginning)
    {
        _scrollRate = (location.y - topScrollBeginning) / scrollZoneHeight;
    }
    else
    {
        _scrollRate = 0;
    }

}

C.如何使用它

在init方法中,将手势识别器分配给表视图.将方法指定为操作longPressGestureRecognized:如下:

    _reorderGestureRecognizer = [[UILongPressGestureRecognizer alloc]
                                               initWithTarget:self action:@selector(longPressGestureRecognized:)];

    [_tableView addGestureRecognizer:_reorderGestureRecognizer];

声明使用上述代码所需的变量

@implementation YourClassName{

    CADisplayLink *_scrollDisplayLink;
    CGFloat _scrollRate;
    UIView *snapshot; // A snapshot of the row user is moving.
    NSIndexPath *sourceIndexPath; // Initial index path, where gesture begins.
}

这就是解决我遇到的问题所需的一切.